* U.S. set to issue weekly harvest progress data

* Mexico buys 1.6 million metric tons of U.S. corn

* Traders monitor dry Australia, Argentina wheat areas

CHICAGO, Sept 25 (Reuters) - Chicago Board of Trade soybean futures neared a seven-week low on Monday and corn fell slightly as U.S. farmers advanced their autumn harvests.

Traders are waiting for the U.S. Department of Agriculture (USDA) to issue a weekly update on harvest progress at 3 p.m. CDT (2000 GMT). Farmers are expected to bring in the nation's second largest corn crop on record.

Some weekend rains in parts of the Midwest were "a speedbump to harvest," said Don Roose, president of Iowa-based broker U.S. Commodities. Still, "harvest is pushing along," he said.

Most-active soybean futures slipped 6-1/4 cents to $12.90 a bushel by 11:50 a.m. CDT (1650 GMT) and touched their lowest price since Aug. 8. Corn futures were down 1/4-cent at $4.77 a bushel, while wheat was up 7-1/4 cents at $5.86-3/4 a bushel.

The USDA, in a daily reporting system, said Mexico bought 1,661,160 metric tons of U.S. corn. Traders await quarterly stocks data due from the USDA on Friday and are also monitoring global wheat values.

Major wheat exporter Australia may see production decline further after hot, dry weather. In Argentina, another global supplier, sales of the upcoming wheat crop are the slowest in seven years, delayed by farmers waiting for rainfall and next month's presidential election.

But large Russian export shipments will limit global prices, analysts said.

"Wheat continues to face downward pressure from huge export supplies from Russia despite harvest downgrades in Australia and Argentina," said Matt Ammermann, StoneX commodity risk manager.

A wheat tender last week from Egypt, a top global buyer, showed offers of Russian wheat all at $270 per metric ton, free on board (FOB) before shipping costs. Traders believe that is a current minimum price sought by the Russian authorities for sales in international tenders. Adherence to that minimum made European Union origins more competitive.

Russian wheat export prices were practically unchanged last week amid weakening global markets, but supply volumes remain high. (Reporting by Tom Polansek in Chicago. Additional reporting by Michael Hogan in Hamburg and Naveen Thukral in Singapore; Editing by Sherry Jacob-Phillips, Janane Venkatraman, Susan Fenton and Barbara Lewis)